The present invention relates to a control system for a direct-injection spark-ignition internal combustion engine and particularly to an ignition timing control at the time of switching of a combustion mode of an engine whose combustion mode is switched in accordance with an operating condition of the engine. The present invention further relates to a method for controlling such an engine.
With a view to improving a fuel consumption of an engine and an exhaust gas purifying ability of a catalytic converter, it is recently used a direct-injection spark-ignition engine in which fuel is injected directly into a combustion chamber to perform stratified combustion.
For promoting activation of a catalytic converter by elevating the exhaust gas temperature, there has been proposed such a direct-injection spark-ignition engine in which fuel is injected in portions dividedly on the intake stroke and on the compression stroke thereby forming closely around a spark plug a stratified mixture of an air-fuel ratio richer than the stoichiometric air-fuel ratio and around the stratified mixture a mixture of an air-fuel ratio leaner than the stoichiometric air-fuel ratio as disclosed in Japanese Patent Provisional Publication No. 10-212987.
In such an engine, the relatively rich mixture around the spark plug is combusted at an initial combustion stage at an elevated combustion speed. Then, combustion proceeds to a main combustion stage in which the relatively lean mixture is combusted. This makes it possible to retard the ignition timing and cause excess fuel to be combusted adjacent the spark plug in a late combustion stage. Such retard of the ignition timing and late combustion of excess fuel makes it possible to suppress emissions of HC, Nox, etc. and elevate the exhaust gas temperature thereby promoting activation of the catalytic converter. In the meantime, in some case, the stratified mixture closely around the spark plug is formed so as to have an air-fuel ratio nearly equal to the stoichiometric air-fuel ratio. Combustion of a mixture formed in the above-described manner is hereinafter referred to as stratified stoichiometric combustion.
In an engine adapted to use stratified stoichiometric combustion when there is a demand for elevating an exhaust gas temperature, homogeneous combustion is used at cold start. The homogeneous combustion is the combustion of a homogeneous mixture formed within the entire combustion chamber and used for attaining stable combustion. Then, the above-described stratified stoichiometric combustion is used to meet with the demand for elevating the exhaust gas temperature. After the catalytic converter has been activated, the combustion mode is switched to homogeneous lean combustion. Further, in accordance with a demand on a operation of the engine, the combustion mode may be switched to stratified lean combustion or homogeneous stoichiometric combustion.
However, since the stratified stoichiometric combustion is lower in the thermal efficiency as compared with the homogeneous combustion that is the combustion of a mixture in which fuel and air are mixed up sufficiently, there occurs a sudden and sharp variation of torque at the time of switching from the stratified combustion to the homogeneous combustion or vise versa, thus causing a problem that the drivability is deteriorated.
To solve this problem, it is considered to correct the ignition timing at the time of switching of the combustion mode thereby suppressing the sudden and sharp variation of torque. For example, at the time of switching from homogeneous combustion to stratified stoichiometric combustion, the sudden and sharp variation of torque can be suppressed by advancing the ignition timing by a predetermined amount simultaneously with the switching of the combustion mode. After the switching of the combustion mode, the ignition timing is gradually retarded to an allowable torque range (stable combustion limit) so that the stratified stoichiometric combustion can produce an exhaust gas temperature elevating effect.
However, in the above-described method (referred to as a comparative example in the drawings), the ignition timing after final retard that meets the allowable torque range is determined based on the operating condition (fuel injection timing, fuel injection quantity) that is determined in consideration of suppression of the sudden and sharp variation of torque at the time of switching of the combustion mode. For this reason, the ignition timing cannot be retarded to an optimum timing in stratified stoichiometric combustion, thus being incapable of maximizing the activation effect of the catalytic converter by an elevated exhaust gas temperature.
It is accordingly an object of the present invention to provide a control system for a direct-injection spark-ignition engine that can eliminate a sudden variation of torque and can maximize the catalyst activation effect by the stratified stoichiometric combustion.
To achieve the above object, there is provided according to an aspect of the present invention a control system for a direct-injection spark-ignition internal combustion engine comprising a first control section for switchably controlling, in accordance with an operating condition of the engine, a combustion mode of the engine between a homogeneous combustion mode wherein a homogeneous air-fuel mixture is formed in an entire combustion chamber by single injection of fuel and a stratified stoichiometric combustion mode wherein a rich air-fuel mixture of an air-fuel ratio equal to or richer than a stoichiometric air-fuel ratio is formed around a spark plug by injection of fuel on a compression stroke and a lean air-fuel mixture of an air-fuel ratio leaner than the stoichiometric air-fuel ratio is formed around the rich air-fuel mixture by injection of fuel on an intake stroke, and a second control section for advancing, at a time of switching from the homogeneous combustion mode to the stratified stoichiometric combustion mode, an ignition timing by an amount for offsetting a variation of torque of the engine simultaneously with the switching of the combustion mode, retarding the ignition timing gradually after the switching of the combustion mode, and retarding, after the ignition timing has been retarded by a predetermined amount, a compression stroke fuel injection timing in the stratified stoichiometric combustion mode while retarding the ignition timing further.
According to another aspect of the present invention, there is provided a method of controlling a direct-injection spark-ignition internal combustion engine comprising switchably controlling, in accordance with an operating condition of the engine, a combustion mode of the engine between a homogeneous combustion mode wherein a homogeneous air-fuel mixture is formed in an entire combustion chamber by single injection of fuel and a stratified stoichiometric combustion mode wherein a rich air-fuel mixture of an air-fuel ratio equal to or richer than a stoichiometric air-fuel ratio is formed around a spark plug by injection of fuel on a compression stroke and a lean air-fuel mixture of an air-fuel ratio leaner than the stoichiometric is formed around the rich air-fuel mixture by injection of fuel on an intake stroke, and advancing, at a time of switching from the homogeneous combustion to the stratified stoichiometric combustion, an ignition timing by an amount for offsetting a variation of torque of the engine simultaneously with switching of the combustion mode, retarding the ignition timing gradually after the switching of the combustion mode, and retarding, after the ignition timing has been retarded by a predetermined amount, a compression stroke fuel injection timing in the stratified stoichiometric combustion mode while retarding the ignition timing further.